1
(Chem.) A vertical column in the periodic table containing elements that resemble each other. The elements of Group 1, the alkali metals, are very alike, being reactive metals which form alkaline hydroxides and ionically bonded compounds exclusively with oxidation number +1. Group 18 contains all the noble gases. For the p-block nonmetals, there are more significant variations within the group, but there remain close similarities. For example, in Group 17, the halogens, the most common oxidation number is -1 in every case and the sodium halides are white crystalline solids which appear almost identical.
The numbering of groups was changed in 1986. Before then the alkali metals were numbered as Group I and the halogens as Group VII. Confusion arose because historically a letter was occasionally added for the p-block elements; UK authors used Group VIIB, and US authors used Group VIIA. To avoid this confusion and to make more explicit use of the knowledge about electronic structure arising from the Schrödinger equation, the groups were renumbered from 1 to 18.See Appendix table 5.
2
(Math.) A mathematical object consisting of a set of elements equipped with an associative binary operation which is not necessarily commutative but for which there is an identity (2) element and inverses for every element.See group theory. |
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